CHICAGO (AP)  Giving teens 30 extra minutes to start their school day leads to more alertness in class, better moods, less tardiness, and even healthier breakfasts, a small study found.

"The results were stunning. There's no other word to use," said Patricia Moss, academic dean at the Rhode Island boarding school where the study was done. "We didn't think we'd get that much bang for the buck."

The results appear in July's Archives of Pediatrics & Adolescent Medicine. The results mirror those at a few schools that have delayed starting times more than half an hour.

Researchers say there's a reason why even 30 minutes can make a big difference. Teens tend to be in their deepest sleep around dawn — when they typically need to get up for school. Interrupting that sleep can leave them groggy, especially since they also tend to have trouble falling asleep before 11 p.m.

"There's biological science to this that I think provides compelling evidence as to why this makes sense," said Brown University sleep researcher Dr. Judith Owens, the study's lead author and a pediatrician at Hasbro Children's Hospital in Providence, Rhode Island.

An Archives editorial said the study adds to "a growing body of evidence that changing the start time for high schools is good for adolescents."

The fact that the study was in the exclusive setting of St. George's School in Middletown, Rhode Island, doesn't necessarily weaken the results. Owens acknowledged that there might be more hurdles to overcome at poorer, public schools, including busing schedules, parents' work hours and daycare for younger siblings.

While these issues have killed many proposals elsewhere, some public high schools including those in Minneapolis and West Des Moines have adopted later starting times.

Mel Riddile, an associate director at the National Association of Secondary School Principals, favors later class times for teens but said most districts oppose it.

"It's about adult convenience, it's not about learning," he said. "With budget cuts, it's going to make it more difficult to get this done."

Many parents and teachers at St. George's were opposed but reluctantly agreed to the study after a presentation by Owens, whose daughter was a junior there.

Overall, 201 high school students completed sleep habit surveys before and after the nine-week experiment last year. The results were so impressive that the school made the change permanent, Moss said.

Starting times were shifted from 8 to 8:30. All class times were cut 5 to 10 minutes to avoid a longer school day that would interfere with after-school activities. Moss said improvements in student alertness made up for that lost instruction time.

The portion of students reporting at least eight hours of sleep on school nights jumped from about 16% to almost 55%. Reports of daytime sleepiness dropped substantially, from 49% to 20%.

First-period tardies fell by almost half, students reported feeling less depressed or irritated during the day, health center rest visits dropped substantially; and the number of hot breakfasts served more than doubled. Moss said the healthier breakfast probably aided classtime alertness.

Recent graduate Garrett Sider, 18, used the extra time for sleep. He noticed kids took part more often in morning classes with the later start time.

"It was a positive thing for the entire school," he said.

The study was designed to look at changes in sleep habits and behavior and didn't examine academic performance. It also lacked a control group of students who didn't experience a change in school start times — another limitation. Still, the researchers said the results show delaying school starting times is worthwhile.

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